Osteochondrosis (OCD) is a degeneration of bone underlying the cartilage layer of joints. It can affect the shoulder, ankle or elbow joint and almost always shows up during the growth phase - between six to nine months of age - of larger breeds. It may start as an intermittent limp in one leg. Many young dogs with OCD run and play as though nothing is wrong but when they slow down they realize the limb hurts and the limp returns. Contributing factors to OCD include both environmental and genetic factors. Dogs whose parents had OCD are much more likely to also suffer from this disease and physical trauma to the joints may also cause the already weakened cartilage to chip and crack.
Full information can be found at this link Pet Education - OCD.